Silicon based MEMS microphones, also known as acoustic transducers, have been in research and development for many years. The silicon based MEMS microphones may be widely used in many applications, such as cell phones, tablet PCs, cameras, hearing aids, smart toys and surveillance devices due to their potential advantages in miniaturization, performances, reliability, environmental endurance, costs and mass production capability.
In general, a silicon based MEMS microphone consists of a fixed perforated backplate and a highly compliant diaphragm with an air gap formed in between. The perforated backplate and the compliant diaphragm, forming a variable air-gap condenser, are typically formed on a single silicon substrate, with one of which being directly exposed to the outside through a back hole formed in the silicon substrate.
Patent application No. WO 02/15636 discloses an acoustic transducer, which has a substrate formed with a back hole therein, a diaphragm made of low stress polysilicon and directly positioned above the back hole of the substrate, and a cover member (equivalent to the said backplate) disposed above the diagram. The diaphragm can be laterally movable within its own plane parallel to the planar surface of the cover member, and thus can release its intrinsic stress, resulting very consistent mechanical compliance.
Patent document PCT/DE97/02740 discloses a miniaturized microphone, in which an SOI substrate is used for formation of the microphone and related CMOS circuits. Specifically, the silicon layer of the SOI substrate is used to form the backplate of the microphone which is directly above a back hole formed in the SOI substrate, and a subsequently deposited polysilicon thin film, which is above the backplate with an air gap in between and is exposed to the outside through the opening in the backplate and the back hole in the SOI substrate, serves to be the diaphragm of the microphone.
When a silicon microphone is packaged, it is usually mounted on a printed circuit board (PCB) with the back hole formed in the substrate of the microphone aligned with an acoustic port formed on the PCB board, so that an external acoustic wave can easily reach and vibrate the diaphragm of the microphone. For example, FIG. 1 shows a cross-sectional view of an exemplary structure of a conventional silicon based MEMS microphone package. As shown in FIG. 1, in the conventional MEMS microphone package, a MEMS microphone 10′ and other integrated circuits 20 are mounted on a PCB board 30 and enclosed by a cover 40, wherein a back hole 140 formed in the substrate 100 of the MEMS microphone 10′ is aligned with an acoustic port 35 formed on the PCB board 30. An external acoustic wave or a sound pressure impact, as shown by the arrows in FIG. 1, travels through the acoustic port 35 on the PCB board 30 and the back hole 140 in the substrate 100 of the microphone 10′ to vibrate the diaphragm 200 of the microphone 10′.
However, as can be seen from the above description, there exists a problem with either the stand-alone conventional MEMS microphones or the conventional MEMS microphone package with the same, which is that the fragile and brittle diaphragm of the conventional MEMS microphones is easily damaged due to a very high sound pressure impact caused, for example, in a drop test.